1. Field of the Invention
The present invention relates to a nonaqueous-electrolyte secondary battery for use as a power supply for a portable electronic device such as a video tape recorder combined with a video camera, a portable telephone set, a laptop computer, or the like
2. Description of the Prior Art
Various portable electronic devices including video tape recorders combined with video cameras, portable telephone sets, laptop computers, etc. have recently been introduced into the market. As efforts are made to reduce the size and weight of those portable electronic devices, attention is attracted to secondary batteries for use as portable power supplies with the portable electronic devices, Research and development activities are being performed to achieve a higher level of energy density for such secondary batteries.
There has been proposed a lithium ion secondary battery which employs a nonaqueous electrolyte having a higher level of energy density than aqueous-electrolyte batteries including lead batteries, nickel-cadmium batteries, etc. Attempts have begun to manufacture lithium ion secondary batteries for practical applications.
Lithium ion secondary batteries include a cylindrical battery having spirally coiled electrodes housed in a cylindrical case and a rectangular battery having folded, rectangular laminated electrodes, or elliptically coiled electrodes housed in a rectangular case. As recent portable electronic devices become thinner, there is a growing demand for the rectangular battery which has a better space utilization efficiency than the cylindrical battery.
The small-size secondary batteries described above are required to be highly reliable in ordinary use and also in high-temperature environments such as automobiles during summer. Rectangular battery cases are liable to be deformed easily under an internal pressure buildup because their strength is smaller than cylindrical battery cases. Therefore, when a rectangular secondary battery which is housed in a portable electronic device is exposed to a high temperature and expanded due to an internal pressure buildup, then the rectangular secondary battery may not be removed from the portable electronic device or may cause damage to the portable electronic device. If such a rectangular secondary battery is designed in smaller dimensions to provide a marginal space therearound, then the energy density that is available from the rectangular secondary battery is so small that the rectangular secondary battery cannot operate for a sufficient period of time.
Lithium ion secondary batteries are charged and discharged when lithium ions are brought into and out of respective crystals of positive and negative electrode materials thereof. When the lithium ions are brought into and out of the crystals, the crystals are expanded and contracted, respectively The cylindrical case of cylindrical lithium ion secondary batteries is strong enough not to be deformed even when the positive and negative electrode materials are expanded. Therefore, when the positive and negative electrode materials are expanded, they are kept in intimate contact with each other, allowing lithium ions to move smoothly for excellent battery characteristics On the other hand, the rectangular case of rectangular lithium ion secondary batteries is easily deformed when the positive and negative electrode materials are expanded Since the deformation of the rectangular case takes place before the positive and negative electrode materials are brought into intimate contact with each other, the rectangular lithium ion secondary batteries fail to provide excellent battery characteristics.